WO2018142968A1

WO2018142968A1 - Electrolyte solution, secondary battery and method for producing electrolyte solution

Info

Publication number: WO2018142968A1
Application number: PCT/JP2018/001585
Authority: WO
Inventors: 和位副田; 石川　正司; 雅紀山縣
Original assignee: 学校法人関西大学
Priority date: 2017-01-31
Filing date: 2018-01-19
Publication date: 2018-08-09
Also published as: US20230089709A1; US20190393555A1; JPWO2018142968A1; JP7112084B2

Abstract

Provided is an electrolyte solution which is capable of maintaining a high magnesium ion concentration. An electrolyte solution according to one embodiment of the present invention is obtained by mixing a solvent, magnesium metal and an elemental halogen.

Description

Electrolytic solution, secondary battery, and method for producing electrolytic solution

The present invention relates to an electrolytic solution, a secondary battery, and a method for producing the electrolytic solution. More specifically, the present invention relates to an electrolyte containing magnesium, a secondary battery, and a method for producing the electrolyte.

In contrast to lithium ion secondary batteries that are currently widely used as secondary batteries, magnesium secondary batteries that use magnesium for the negative electrode are attracting attention as “next-generation secondary batteries”. Magnesium secondary batteries have advantages over lithium ion secondary batteries in that they have a large amount of electricity available per volume, are rich in metal resources, are inexpensive, and are highly safe in the air. It is.

In a magnesium secondary battery, metallic magnesium dissolves as magnesium ions during discharge, and conversely, magnesium ions precipitate as metallic magnesium during charging. For this reason, the electrolyte solution for a magnesium secondary battery is required to have a characteristic of retaining a sufficient amount of magnesium ions and efficiently causing a dissolution reaction and a precipitation reaction with the negative electrode (metallic magnesium). .

As an example of such an electrolytic solution, Patent Document 1 discloses an electrolytic solution in which a magnesium salt is dissolved in a solvent made of sulfone. The literature also describes, as a method for producing the electrolytic solution, (1) a step of dissolving a magnesium salt in a low-boiling solvent (for example, alcohol) in which the magnesium salt can be dissolved, (2) a solution obtained in (1) And (3) a step of removing the low boiling point solvent from the solution obtained in (2).

Patent Document 2 discloses an ion conductive medium containing magnesium ions, a halogen, and a non-aqueous solvent. According to this document, halogen and a non-aqueous solvent form a molecular complex in the ion conductive medium.

Japanese Patent Publication “Japanese Patent Laid-Open No. 2014-072031 (published on April 21, 2014)” Japanese Patent Publication “JP 2013-037993 A (published on February 21, 2013)”

However, the conventional techniques as described above have room for improvement in terms of battery output because the magnesium ion concentration in the electrolytic solution cannot be sufficiently increased.

An object of one embodiment of the present invention is to provide an electrolytic solution capable of maintaining a high magnesium ion concentration.

The present inventors have found that the above problem can be solved by an electrolytic solution produced by a method in which metallic magnesium and simple halogen are dissolved in a solvent. That is, the present invention includes the following.

<1>
An electrolytic solution obtained by mixing a solvent, magnesium metal and simple halogen.

<2>
The electrolyte solution according to <1>, which contains a solvent, magnesium ions, and halide ions.

<3>
The electrolytic solution according to <1> or <2>, which contains 0.5 mol / L or more of magnesium ions with respect to the total amount of the electrolytic solution.

<4>
In any one of <1> to <3>, the magnesium atom having a coordination number of 4 accounts for 95% or more of the total number of magnesium atoms when analyzed by the soft X-ray fluorescence XAFS method. Electrolyte of description.

<5>
The electrolyte solution according to any one of <1> to <4>, wherein the solvent is an organic solvent or an ionic liquid.

<6>
The electrolyte solution according to any one of <1> to <5>, wherein the solvent is a sulfone solvent.

<7>
The electrolyte solution according to any one of <1> to <6>, which contains bromide ions or iodide ions as halide ions.

<8>
A secondary battery comprising the electrolytic solution according to any one of <1> to <7>.

<9>
The manufacturing method of electrolyte solution including the process of mixing a solvent, metallic magnesium, and a single-piece | unit halogen.

<10>
<9> The method for producing an electrolytic solution according to <9>, wherein magnesium ion is contained in an amount of 0.5 mol / L or more with respect to the total amount of the electrolytic solution.

<11>
<9> or <10>, wherein when the electrolyte solution is analyzed by a soft X-ray fluorescence XAFS method, magnesium atoms having a coordination number of 4 occupy 95% or more of the total number of magnesium atoms The manufacturing method of the electrolyte solution.

<12>
The method for producing an electrolytic solution according to any one of <9> to <11>, wherein the solvent is an organic solvent or an ionic liquid.

<13>
The method for producing an electrolytic solution according to any one of <9> to <12>, wherein the solvent is a sulfone solvent.

<14>
The method for producing an electrolytic solution according to any one of <9> to <13>, wherein the single halogen is a bromine molecule or an iodine molecule.

According to one embodiment of the present invention, an electrolytic solution that can maintain a high magnesium ion concentration is provided.

(A) is a cyclic voltammogram showing the cyclic voltammetry measurement result of the electrolyte solution which concerns on one Embodiment of this invention. (B) is a cyclic voltammogram showing the cyclic voltammetry measurement result of the electrolyte solution described in Patent Document 1. (A) is the electron microscope image which image | photographed the metal magnesium deposit produced by sending an electric current through the electrolyte solution which concerns on one Embodiment of this invention. The bright part is metal magnesium deposited, and the dark part is nickel of the substrate. (B) is an element mapping image obtained by analyzing the same range as (a) by energy dispersive X-ray analysis (EDX) and color-coding each element. (C) shows the analysis result in (b) as a mapping spectrum. It is a graph showing the result of having analyzed the electrolyte solution (solid line) which concerns on one Embodiment of this invention, and the magnesium perchlorate aqueous solution (broken line) by the soft X ray fluorescence XAFS method. In addition, the broken line is a reference curve representing the analysis result when the solvent molecule is 6-coordinated to the magnesium atom. (A) shows the progress of the initial stage (1st cycle to 10th cycle) when imposing a constant current charge / discharge test on the electrolytic solution according to one embodiment of the present invention under the conditions assuming the operation as a battery. It is a graph to represent. (B) is a graph showing the course of the final stage of the same test as in (a) (cycles 5011 to 5014). 4th cycle, 400th cycle, 600th cycle, 800th cycle, 1000th cycle when the constant current charge / discharge test is imposed on the electrolyte solution according to the embodiment of the present invention under conditions different from those in FIG. It is a graph showing the behavior of 1100th cycle. The first cycle when a constant current charge / discharge test is imposed on a secondary battery produced by using an electrolytic solution according to an embodiment of the present invention as an electrolytic solution, vanadium pentoxide as a positive electrode, and metallic magnesium as a negative electrode, It is a graph showing the behavior of the 5th cycle, the 10th cycle, and the 20th cycle. (A)-(c) is a cyclic voltammogram showing the cyclic voltammetry measurement result of the electrolyte solution which concerns on other embodiment of this invention. Each type of solvent is changed. (A)-(d) is a cyclic voltammogram showing the cyclic voltammetry measurement result of the electrolyte solution which concerns on other embodiment of this invention. In each case, the types of simple halogen and solvent are changed.

An embodiment of the present invention will be described as follows, but the present invention is not limited to this. This invention is not limited to each structure demonstrated below, A various change is possible in the range shown to the claim. Therefore, embodiments and examples obtained by appropriately combining technical means disclosed in different embodiments and examples are also included in the technical scope of the present invention. Moreover, all the literatures described in this specification are used as a reference in this specification.

In this specification, when “A to B” is described with respect to a numerical range, the description is intended to be “A to B”.

The electrolytic solution according to an embodiment of the present invention is an electrolytic solution obtained by mixing a solvent, metallic magnesium, and a single halogen. Moreover, the electrolyte solution which concerns on one Embodiment of this invention is also an electrolyte solution containing a solvent, magnesium ion, and halide ion. Hereinafter, each component will be described in order.

[1. solvent〕
If the solvent which the electrolyte solution which concerns on one Embodiment of this invention contains is a solvent normally used when manufacturing electrolyte solution, it will not specifically limit. Examples of such solvents include organic solvents and ionic liquids.

Specific examples of the organic solvent include sulfone solvents (dimethylsulfone, methylisopropylsulfone, ethylmethylsulfone, ethylisopropylsulfone, ethylisobutylsulfone (EiBS), dipropylsulfone, isopropyl-s-butylsulfone (iPsBS), isopropyl Isobutyl sulfone (iPiBS), butyl isobutyl sulfone (BiBS), sulfolane, etc.), ether solvents (2-methyltetrahydrofuran, dimethoxyethane, dioxolane, monoglyme (G1), diglyme (G2), triglyme (G3), tetraglyme (G4) ) Etc.). Examples of other ether solvents include a mixed solvent of dimethoxyethane and dioxolane. From the viewpoint of excellent electrochemical properties of the electrolytic solution, among the sulfone solvents, methyl isopropyl sulfone, ethyl isopropyl sulfone, dipropyl sulfone, and sulfolane are preferable, and ethyl isopropyl sulfone is more preferable. From the same viewpoint, among the ether solvents, 2-methyltetrahydrofuran is preferable.

Examples of the ionic liquid include DEMETFSI (Dimethylmethyl (2-methoxyethyl) ammoniumbis (trifluoromethylsulfonyl) imide), DEMEBF4 (Diethylmethyl (2-methoxyethyl) ammoniumtetrafluoroborate), EMIBF4 (1-Ethyl-3-methylimidazoliumtetrafluoroborate), EMITFSI (1-Ethyl -3-methylimidazolium (bis (trifluoromethanesulfonyl) imide)) EMISI (1-Ethyl-3-methylimidazolium (bis (fluorosulfonyl) imide)) and the like. From the viewpoint that the electrochemical properties of the electrolytic solution are excellent, DEMETFSI is preferable among the ionic liquids.

Among the solvents described above, an organic solvent or an ionic liquid is preferable from the viewpoint that the electrochemical properties of the electrolytic solution are excellent. In consideration of the fact that the magnesium dissolution reaction and precipitation reaction that accompany charging and discharging can be repeated, sulfone solvents and ether solvents are more preferable. In consideration of the low volatility, the low toxicity, and the permissible water content, the sulfone solvent is more preferable.

Therefore, among the solvents exemplified above, methyl isopropyl sulfone, ethyl isopropyl sulfone, dipropyl sulfone, and sulfolane are particularly preferable, and ethyl isopropyl sulfone is further particularly preferable.

The solvent mentioned above may contain only one type, or two or more types.

[2. Magnesium ion)
The electrolytic solution according to one embodiment of the present invention contains magnesium ions. In the magnesium secondary battery, the metallic magnesium of the negative electrode is dissolved in the electrolytic solution as magnesium ions during discharge, and the magnesium ion in the electrolytic solution is deposited on the negative electrode as metallic magnesium during charging. For this reason, in order to increase the battery output, it is necessary to sufficiently increase the magnesium ion concentration in the electrolytic solution.

The magnesium ion concentration in the electrolytic solution according to an embodiment of the present invention is preferably 0.50 mol / L or more, more preferably 0.55 mol / L or more, and 0.75 mol / L with respect to the total amount of the electrolytic solution. Further preferred. If the magnesium ion concentration in the electrolytic solution is 0.50 mol / L or more, sufficient battery output can be obtained when the battery is manufactured.

On the other hand, considering the time and cost required for production, the magnesium ion concentration in the electrolytic solution according to an embodiment of the present invention is suitably about 1.00 mol / L or less with respect to the total amount of the electrolytic solution. However, this value is merely an example and is not intended to limit the present invention.

In the electrolytic solution containing magnesium ions, the coordination number of solvent molecules with respect to one magnesium atom is preferably 4. A magnesium atom in such a state has (1) higher solubility in a solvent and higher magnesium ion concentration than a magnesium atom having a coordination number of 6 solvent molecules to one magnesium atom. (2) It is excellent in that the reaction activity of magnesium atom is high and the dissolution and precipitation of magnesium accompanying charge / discharge is easy to proceed (for the above phenomenon, see [Saha P et. Al (2014) "Rechargeable" magnesium "battery:" Currentstatus "and" key "challenges" for "the" future "," Progress "in" Materials "Science, Vol.66," pp.1-86]).

The coordination number of the solvent with respect to the magnesium atom in the electrolytic solution can be known, for example, by the soft X-ray fluorescence XAFS method (for details, see Example 3). When the peak intensity corresponding to the first adjacent atom with respect to the magnesium atom is measured, the proportion of the magnesium atom having the coordination number of 4 can also be calculated.

Here, the soft X-ray fluorescence XAFS method is a technique for irradiating a sample with soft X-rays and measuring and analyzing the fluorescent X-rays emitted secondarily. The structure of a specific atom in a sample can be known by analyzing the fluorescent X-ray and obtaining a radial structure function. For more detailed theories and methodologies, see, for example, “Yasuo Udagawa,“ X-ray absorption fine structure: XAFS measurement and analysis ”, Society of Science Publishing Center, 1993; Toshiaki Ota,“ X-ray absorption spectroscopy: XAFS and its applications ” PC, 2002].

In the electrolytic solution according to an embodiment of the present invention, it is preferable that magnesium atoms having a coordination number of 4 occupy 95% or more of the total magnesium atoms when analyzed by the soft X-ray fluorescence XAFS method. More preferably 97% or more, and even more preferably 99% or more. If magnesium atoms having a coordination number of 4 occupy 95% or more of all magnesium atoms, it can be said that the solubility of magnesium in the solvent and the reaction activity of magnesium atoms are sufficiently high.

On the other hand, considering the time and cost required for production, in the electrolytic solution according to one embodiment of the present invention, when analyzed by the soft X-ray fluorescence XAFS method, About 99.9% or less of the magnesium atom is appropriate. However, this value is merely an example and is not intended to limit the present invention.

[3. (Halide ion)
The electrolytic solution according to one embodiment of the present invention contains halide ions. From the viewpoint of not corroding the magnesium electrode, it is preferable that no non-ionized single halogen exists in the solvent. In addition, according to the manufacturing method which concerns on one Embodiment of this invention mentioned later, since a single-piece | unit halogen is reduce | restored by metallic magnesium in a solvent, it will exist in the state of a halide ion.

In this specification, “single halogen” means a fluorine molecule (F ₂ ), a chlorine molecule (Cl ₂ ), a bromine molecule (Br ₂ ), or an iodine molecule (I ₂ ). In this specification, “halide ion” means fluoride ion (F ⁻ ), chloride ion (Cl ⁻ ), bromide ion (Br ⁻ ) or iodide ion (I ⁻ ).

From the viewpoint of the stability of the single halogen and halide ions, the halide ions dissolved in the electrolyte solution according to one embodiment of the present invention are preferably bromide ions or iodide ions, and more preferably iodide ions.

The concentration of halide ions dissolved in the electrolytic solution according to one embodiment of the present invention is not particularly limited. From the viewpoint of improving the production yield of the electrolytic solution, the concentration of halide ions dissolved in the electrolytic solution according to one embodiment of the present invention is preferably 0.5 mol / L or more, and 0.55 mol / L or more. Is more preferable, and 0.75 mol / L or more is more preferable.

On the other hand, considering the time and cost required for production, the concentration of halide ions in the electrolytic solution according to one embodiment of the present invention is suitably about 0.90 mol / L or less with respect to the total amount of the electrolytic solution. is there. However, this value is merely an example and is not intended to limit the present invention.

Only one type of halide ion described above may be included, or two or more types may be included.

[4. Other ingredients]
The electrolytic solution according to one embodiment of the present invention may contain components other than those described above. Examples of such components include Lewis bases (such as magnesium ethoxide).

[5. Production method〕
The manufacturing method of the electrolyte solution which concerns on one Embodiment of this invention includes the process of mixing a solvent, metallic magnesium, and single-piece | unit halogen. The order in which each component is mixed is not particularly limited. That is, the solvent and metal magnesium may be mixed first, the solvent and single halogen may be mixed first, or the metal magnesium and single halogen may be mixed first. Moreover, you may mix three components with a solvent simultaneously. From the viewpoint of improving the production yield of the electrolytic solution, it is preferable to mix the simple halogen and the solvent first, and then mix the magnesium metal and the solvent. In addition, the manufacturing method of the electrolyte solution which concerns on one Embodiment of this invention may also include another process.

According to the method for producing an electrolytic solution according to one embodiment of the present invention, a redox reaction using metallic magnesium as a reducing agent and single halogen as an oxidizing agent occurs in a solvent, and magnesium ions are generated. This is greatly different from a conventional manufacturing method (for example, a manufacturing method described in Patent Document 1) in which a magnesium salt is dissolved in a solvent. The present inventors have found that an electrolytic solution having a magnesium concentration higher than that of the conventional manufacturing method can be manufactured by the above manufacturing method, and have completed the present invention.

In order to prevent reaction with oxygen, nitrogen, moisture, and organic solvent volatiles, it is preferable to measure the single halogen and mix the single halogen and the solvent in an inert gas atmosphere. Such an environment can be obtained, for example, by using a glove box.

[5-1. Metal magnesium]
The magnesium metal used in the method for producing an electrolytic solution according to an embodiment of the present invention is a metal containing magnesium as a main component (for example, a metal in which magnesium accounts for 95% by weight or more with 100% by weight of the total weight of the metal) If there is, the purity is not particularly limited. From the viewpoint of minimizing unexpected reactions due to impurities, the purity of the metallic magnesium is preferably 96% by weight or more, more preferably 98% by weight or more, and further preferably 99.9% by weight or more.

On the other hand, considering the time and cost required for production, the purity of the metal magnesium is suitably about 99.99% by weight or less. However, this value is merely an example and is not intended to limit the present invention.

The amount of magnesium metal used in the method for producing an electrolytic solution according to one embodiment of the present invention is preferably large excess (for example, 4 times or more the weight of the single halogen) with respect to the single halogen. By making the amount of metallic magnesium large excess with respect to the amount of single halogen, the single halogen can be completely reacted and corrosion of the electrode by the single halogen remaining in the solvent can be prevented.

[5-2. Single halogen]
The single halogen used in the method for producing an electrolytic solution according to one embodiment of the present invention is as described in [3].

If the single halogen used in the method for producing an electrolyte solution according to an embodiment of the present invention is a single halogen as a main component (for example, the total weight including impurities is 100 wt%, the single halogen occupies 99 wt% or more). The purity is not particularly limited. From the viewpoint of minimizing unexpected reactions due to impurities, the single-body halogen purity is preferably 99% by weight or more, more preferably 99.9% by weight or more, and even more preferably 99.99% by weight or more.

On the other hand, considering the time and cost required for production, the purity of the single halogen is suitably about 99.999% by weight or less. However, this value is merely an example and is not intended to limit the present invention.

In addition, in the manufacturing method of the electrolyte solution which concerns on one Embodiment of this invention, single type | mold halogen may use only 1 type and may use 2 or more types together.

[5-3. solvent]
The solvent used in the method for producing an electrolytic solution according to one embodiment of the present invention is as described in [1].

[5-4. Example of manufacturing method]
The manufacturing method of the electrolyte solution which concerns on one Embodiment of this invention is as follows, for example. A more specific example of the production method is described in [Production Example] below.

First, halogen is added to a solvent and dissolved (or dispersed). Metal magnesium is then added to the solution. By stirring the obtained solution under atmospheric pressure, an oxidation-reduction reaction occurs between the halogen in the solvent and the magnesium metal. By sufficiently allowing the above reaction to proceed (about 10 to 24 hours), the electrolytic solution according to one embodiment of the present invention can be produced.

[6. Secondary battery)
A secondary battery can be produced by combining a positive electrode and a negative electrode with the electrolytic solution according to one embodiment of the present invention. The material, shape, and the like of the positive electrode, the negative electrode, and other members (separator, etc.) can be appropriately selected by those skilled in the art. An example of a more specific manufacturing method is described in [Example 5] below.

When producing a magnesium secondary battery, the material of the negative electrode is usually metallic magnesium. Examples of the material for the positive electrode include vanadium pentoxide, molybdenum sulfide, magnesium-containing oxide, and transition metal oxide.

Moreover, since the electrolytic solution according to an embodiment of the present invention allows moisture to be contained, an air secondary battery can also be manufactured. In this case, the material of the negative electrode is usually metallic magnesium. Examples of the positive electrode material (including the catalyst layer) include, in addition to carbon (C), platinum (Pt), ruthenium (Ru), iridium (Ir), rhodium (Rh), palladium (Pd), and osmium (Os). ), Tungsten (W), lead (Pb), iron (Fe), chromium (Cr), cobalt (Co), nickel (Ni), manganese (Mn), vanadium (V), molybdenum (Mo), gallium (Ga) ), Metals such as aluminum (Al) and their compounds, and alloys thereof. Among these, when carbon is employed as the amount of the positive electrode material, for example, a secondary battery that does not use a rare metal can be manufactured.

[7. Advantages over conventional technology
Differences from the prior art will be described below with respect to one aspect considered to be the most effective of the present invention. The above embodiment is an electrolytic solution prepared by dissolving a simple halogen in a sulfone solvent and then mixing a sufficient amount of metallic magnesium so that the simple halogen is completely reacted. However, it should be noted that other embodiments may be optimal depending on various conditions and purposes.

[Compatibility of high magnesium ion concentration with low toxicity and moisture tolerance]
Conventional magnesium ion-containing electrolytes have been developed by employing ether solvents or sulfone solvents as solvents. Among these, although the electrolyte solution using an ether solvent can achieve a high magnesium ion concentration, it has a drawback that it becomes inactive because it has high toxicity and volatility and the coordination structure is destroyed when it contains water. For this reason, the electrolytic solution using an ether solvent has a problem in practical use. On the other hand, an electrolyte solution using a sulfone solvent has low toxicity and low volatility and is allowed to contain a certain amount of moisture, but it has been difficult to increase the magnesium ion concentration.

In the prior art, magnesium ions in the solvent are generated by dissolving the magnesium salt. However, since the solubility of the magnesium salt in the sulfone solvent is low, it is necessary to use an auxiliary solvent and / or an additive together (for example, Patent Document 1 uses alcohol as the auxiliary solvent).

On the other hand, according to the above-described embodiment, a high magnesium ion concentration exceeding 0.50 mol / L can be achieved while using a sulfone solvent that can withstand practical use (long battery life). Further, since magnesium ions are generated without using an auxiliary solvent and / or an additive, the number of production steps can be reduced.

[Reduction of overvoltage]
In general, a magnesium secondary battery has a problem (overvoltage) in that reaction resistance occurs and no current is generated even when a potential difference occurs where magnesium is dissolved or precipitated. As a technique for solving this problem, there is a technique in which a single halogen coexists in an electrolytic solution (for example, Patent Document 2). However, the above method still has a problem that single halogen corrodes the magnesium electrode.

On the other hand, in the above embodiment, corrosion of the electrode does not occur because the single halogen completely reacts in the manufacturing stage. In addition, as shown in the examples described later, the overvoltage hardly occurs or succeeds in stopping at a small value.

[High actual capacity]
In the above embodiment, a high actual capacity is recorded as the magnesium electrode / magnesium-containing electrolyte system. The theoretical capacity of the magnesium electrode / magnesium-containing electrolyte system (the amount of electricity that can be taken out when all the magnesium electrodes are dissolved and deposited) is 3881 mAh / cm ³ . According to the above aspect, an actual capacity equivalent to 50% utilization (1941 mAh / cm ³ ) could be achieved with a cycle efficiency of 99.8% (see Example 4-1).

[8. Other Embodiments of the Present Invention]
In another aspect, the present invention includes the following configurations.
<1> An electrolytic solution containing a solvent, magnesium ions, and halide ions.
<2> The electrolyte solution according to <1>, which contains 0.5 mol / L or more of magnesium ions with respect to the total amount of the electrolyte solution.
<3> The magnesium atom having a coordination number of 4 accounts for 95% or more of the total number of magnesium atoms when analyzed by the soft X-ray fluorescence XAFS method, according to <1> or <2> Electrolytic solution.
<4> The electrolytic solution according to any one of <1> to <3>, wherein the solvent is an organic solvent or an ionic liquid.
<5> The electrolytic solution according to any one of <1> to <4>, wherein the solvent is a sulfone solvent.
<6> The electrolytic solution according to any one of <1> to <5>, wherein the halide ion is a bromide ion or an iodide ion.
<7> A secondary battery comprising the electrolytic solution according to any one of <1> to <6>.
<8> A method for producing an electrolytic solution, comprising a step of mixing a solvent, metallic magnesium, and simple halogen.
<9> The method for producing an electrolytic solution according to <8>, wherein magnesium ions are contained in an amount of 0.5 mol / L or more with respect to the total amount of the electrolytic solution.
<10> When the electrolyte is analyzed by a soft X-ray fluorescence XAFS method, magnesium atoms having a coordination number of 4 occupy 95% or more of the total number of magnesium atoms, <8> or <9 The manufacturing method of electrolyte solution as described in>.
<11> The method for producing an electrolytic solution according to any one of <8> to <10>, wherein the solvent is an organic solvent or an ionic liquid.
<12> The method for producing an electrolytic solution according to any one of <8> to <11>, wherein the solvent is a sulfone solvent.
<13> The method for producing an electrolytic solution according to any one of <8> to <12>, wherein the single halogen is a bromine molecule or an iodine molecule.

[Production example]
An electrolytic solution according to an embodiment of the present invention was prepared by the following method. The measurement of the reagent and the mixing of the reagent and the solvent were performed in a glove box (argon atmosphere, dew point: −80 to −90 ° C.).

As a solvent, 10 mL of ethyl isopropyl sulfone (manufactured by Tokyo Chemical Industry) dehydrated by molecular sieve was weighed. While stirring the above solvent with a stirrer, 1.27 g of iodine (manufactured by Wako Pure Chemical Industries, Ltd.) was added. After iodine was completely dispersed in the solvent, 0.486 g of metal magnesium powder was added. It was observed that the purple color derived from iodine faded as the reaction proceeded. About 12 hours after adding the magnesium metal powder, the reaction was judged complete when the purple color disappeared completely and the solution became transparent. The above solution was taken out of the glove box while maintaining a state in which air was not mixed, and centrifuged (7500 rpm, 15 minutes) with a centrifuge (AS165W, manufactured by ASONE). Thus, an electrolyte solution according to an embodiment of the present invention (hereinafter referred to as “electrolyte solution A”) was obtained as a supernatant liquid in which unreacted metallic magnesium was precipitated.

The magnesium concentration of the prepared electrolytic solution A was 0.55 mol / L. In addition, it was confirmed that the magnesium concentration was increased to 3.5 mol / L at the maximum by the same manufacturing method.

[Example 1]
In order to examine the electrochemical characteristics of the electrolyte A, cyclic voltammetry (CV) measurement was performed.

For the measurement, a triode cell (electrolyte volume 0.7 mL; manufactured by BAS, VC-4) was used. A nickel (Ni) substrate (diameter 10 mm) was used as a working electrode, and magnesium (Mg) pellets (diameter 12 mm; manufactured by Rare Metallic) and magnesium wires (diameter 1.6 mm; manufactured by Rare Metallic) were used as a counter electrode and a reference electrode. The measurement was performed at room temperature and atmospheric pressure.

Measurement was performed by sweeping the potential in the following cycle.

(1) At the start, the electrodes were placed in an open circuit state (OCV).

(2) First, the potential of the working electrode with respect to the potential of the reference electrode was decreased to −0.7 V toward the reduction side. During this time, magnesium ions precipitated as metallic magnesium.

(3) Next, the potential of the working electrode with respect to the potential of the reference electrode was increased to 2.0 V toward the oxidation side. During this time, magnesium metal was dissolved as magnesium ions.

(4) Finally, the electrode gap was returned to OCV.

That is, the potential of the working electrode with respect to the reference electrode was changed in the order of OCV → −0.7V → about +1.0 to 2.0V → OCV. The speed at which the potential was inserted was 20 mV / s.

(result)
A cyclic voltammogram representing the result is shown in FIG. According to the figure, when the potential is swept in the negative direction, a response current is generated from the time of 0 V (see point A). This indicates that magnesium precipitation actually starts at a theoretical potential at which magnesium precipitation begins thermodynamically. That is, it shows that the precipitation reaction of magnesium proceeds without causing overvoltage. Similarly, when the potential is swept in the positive direction, it can be read from FIG. 1A (see point B) that the magnesium dissolution reaction proceeds without causing overvoltage.

On the other hand, a cyclic voltammogram of the electrolytic solution described in Patent Document 1 which is a prior art is shown in FIG. The above electrolyte was prepared by dissolving magnesium chloride (II) in dehydrated methanol, mixing it with ethyl-n-propylsulfone (EnPS), and removing the methanol under reduced pressure.

According to FIG. 1 (b), the response current when the potential is swept in the negative direction is generated from around −1V (see point C). That is, a reaction resistance is generated between 0V and 1V, and the precipitation of magnesium does not start (overvoltage is generated). Similarly, when the potential is swept in the positive direction, a slight overvoltage is observed (see point D). This difference is derived from the fact that in the electrolytic solution described in Patent Document 1, magnesium atoms having six coordinated solvent molecules and magnesium atoms having four coordinated solvent molecules are mixed. Conceivable.

In addition, the current density in FIG. 1A is on the order of A / cm ² , whereas the current density in FIG. 1B is on the order of mA / cm ² . That is, the electrolytic solution A has succeeded in extracting a current about 1000 times that of the prior art.

[Example 2]
From the electrolytic solution A, it was confirmed that metallic magnesium was deposited.

[Example 2-1]
It was confirmed by electron microscopy that metallic magnesium was deposited.

Electrolytic solution A was injected into a triode cell (electrolytic solution amount 2.0 mL; manufactured by BAS, VC-4), a nickel electrode was inserted into the working electrode, and magnesium metal was inserted into the counter electrode and the reference electrode. Next, a current of 1 mA / cm ² was passed between the working electrode and the counter electrode for 10 minutes. Thereafter, the working electrode to which the deposit was attached was immersed in EiPS, washed, dried under reduced pressure, and then the deposit on the working electrode was observed with a scanning electron microscope (manufactured by Hitachi hightech). The acceleration voltage of the electron gun filament was 15 kV, and the current was 40.0 mA.

(result)
The photographed electron microscope image is shown in FIG. Precipitated magnesium metal (bright part) is observed at the center of the screen. The dark part is the working electrode nickel.

[Example 2-2]
Energy dispersive X-ray analysis (EDX) was performed using an energy dispersive X-ray analyzer (Genesis XM2 manufactured by Edax Japan) to analyze the elements of the precipitate.

(result)
The mapping analysis result for the same range as in FIG. 2A is shown in FIG. Thereby, it was shown that (a) of FIG. 2 is the metallic magnesium deposited on the nickel substrate. The mapping spectrum of the precipitate is shown in FIG. It turns out that the spectrum peculiar to magnesium is detected strongly. In addition, S is a sulfone solvent and I is a spectrum derived from an electrolytic solution. From these results, it was shown that metallic magnesium was deposited on the electrode.

Example 3
The electrolyte solution A was analyzed by the soft X-ray XAFS method, and the state of magnesium atoms contained therein was investigated.

Electrolyte A0.2mL was poured into a stainless steel sample holder with a Be window and analyzed with a soft X-ray XAFS analysis facility (High Energy Accelerator Research Organization Photon Factory, BL-11). The analyzed energy region was 1250-1550 eV. Magnesium K absorption was collected from the obtained fluorescent X-rays and analyzed by data processing software ATHENA to obtain a radial structure function.

As a comparative sample, an aqueous magnesium perchlorate solution was analyzed by a soft X-ray XAFS method to obtain a radial structure function. The magnesium perchlorate aqueous solution was prepared by dissolving magnesium perchlorate (manufactured by Wako Pure Chemical Industries, Ltd.) in water to 0.55 M and stirring with a stirrer in the atmosphere. In addition, it is known that the magnesium perchlorate aqueous solution will coordinate 6 water molecules with respect to a magnesium atom.

(result)
The radial structure functions of the electrolytic solution A (solid line) and the magnesium perchlorate aqueous solution (broken line) are shown in FIG. The horizontal axis of the graph represents the distance from the center of the magnesium atom, and the peak size of the radial structure function is proportional to the number of atoms. In a magnesium perchlorate aqueous solution, it is known that water molecules are six-coordinated to magnesium atoms, and therefore, the broken line can be regarded as a function in the case of six-coordinates.

Comparing the intensities of the peaks corresponding to the first adjacent atoms (peaks near 3Å) of the respective radial structure functions, the ratio between the peak intensity of the electrolyte A and the peak intensity of the magnesium perchlorate aqueous solution is 4. 015: 6.00. That is, in the electrolytic solution A, four atoms are present in the vicinity of most of the magnesium atoms. This suggests that the sulfone solvent molecule is 4-coordinated around the magnesium atom.

Assuming that the coordination number of the solvent molecule to the magnesium atom is either tetracoordinate or hexacoordinate, 99.25% of the total magnesium atom is tetracoordinate and 0.75% is It will be 6-coordinated. As described above, since tetracoordinate magnesium has high solubility in a solvent and high reaction activity, it can be said that the electrolytic solution A is a preferable state as an electrolytic solution.

Example 4
[Example 4-1]
A constant current charge / discharge test was imposed on the electrolytic solution A under conditions assuming operation as a battery.

Electrolyte solution A0.5mL was poured into a bipolar cell (made by Hosen), and a nickel electrode was inserted into the working electrode and metallic magnesium was inserted into the counter electrode. Prior to repeated charging and discharging, precharging at 10 C / cm ² was performed to deposit metallic magnesium. Then, (1) the current 1.0 mA / cm ² flowed 500 seconds toward the working electrode to the counter electrode, (2) passing a current of 1.0 mA / cm ² for 500 seconds toward the working electrode to the counter electrode, it Repeated charging and discharging. Note that the current flowing under these conditions is a current corresponding to repeating dissolution and precipitation in 5% of the metal magnesium initially precipitated. The charge / discharge cycle was repeated until dissolution and precipitation of metallic magnesium could not be repeated, and the cycle efficiency was calculated according to the following formula.

In the formula, N represents the number of cycles. x represents the amount of electricity (C / cm ² ) per unit area charged by the preliminary charging. y (charge) represents the amount of electricity (C / cm ² ) used to deposit Mg on the working electrode, and y (discharge) represents the amount of electricity (C / cm ² ) used to dissolve Mg deposited on the working electrode. cm ² ).

(result)
The graph showing progress was shown to (a) and (b) of FIG. Where the potential is constant, magnesium dissolution or precipitation occurs. In this test, magnesium could be dissolved or precipitated until reaching 5000 cycles. The cycle efficiency calculated from this result is 99.8%. Similarly, the actual capacity calculated from this result was 1940 mAh / cm ³ . In addition, the overvoltage which generate | occur | produced in this test was about 4 mV.

[Example 4-2]
A constant current charge / discharge test was imposed on the electrolytic solution A under other conditions assuming operation as a battery. Specifically, a constant current charge / discharge test was imposed under the same conditions as in Example 4-1, except that the current flow time was changed to 5000 seconds. The current flowing under these conditions is a current corresponding to repeating dissolution and precipitation in 50% of the metal magnesium initially precipitated. In this example, the number of charge / discharge cycles was 1100, and the cycle efficiency was adjusted to be equivalent to that of Example 4-1.

(result)
FIG. 5 shows the progress of the 200th cycle, 400th cycle, 600th cycle, 800th cycle, 1000th cycle, and 1100th cycle. From the figure, it can be seen that the dissolution and precipitation behavior of magnesium is stable even when the charge / discharge cycle is repeated. The actual capacity when reaching 1100 cycles was 50 mAh / cm ³ . The overvoltage generated in this test was about 1V.

Example 5
The electrolyte solution A, the negative electrode, and the positive electrode were combined to produce a magnesium secondary battery. Furthermore, a constant current charge / discharge test was imposed on the magnesium secondary battery.

[Production of battery]
A coin battery was manufactured using magnesium (Mg) for the negative electrode, vanadium pentoxide (V ₂ O ₅ ) for the positive electrode, and electrolytic solution A for the electrolytic solution. The method for producing the coin battery is as follows. Place the gasket coin battery can further (Mg pellets thickness 200 [mu] m), (stainless steel plate having a thickness of 500 [mu] m) spacer its cathode on _(V 2 _{O 5} pellets of thickness 30 [mu] m), polyolefin separator, a negative electrode, the washer The coin battery lids were stacked in this order. Thereafter, 100 μL of electrolyte solution A was injected, and the coin battery can was crimped and sealed.

[Constant current charge / discharge test]
For the magnesium secondary battery manufactured by the above-described method, (1) charging at a current of 1.0 mA / cm ² for 3600 seconds, (2) discharging at a current of 1.0 mA / cm ² for 3600 seconds, 20 The cycle was repeated.

(result)
The progress of the 1st cycle, 5th cycle, 10th cycle, and 20th cycle is shown in FIG. The positive electrode of the battery manufactured in this example has magnesium insertion reaction: V ₂ O ₅ + nMg ²⁺ + 2ne ⁻ → Mg _n V ₂ O ₅ and magnesium elimination reaction: Mg _n V ₂ O ₅ → V ₂ O _5. + NMg ²⁺ + 2ne ⁻ is repeated. From FIG. 6, it can be seen that even when the charge / discharge cycle is repeated, curves corresponding to the insertion and desorption reactions of magnesium are obtained, and the reaction is stable.

Example 6
[Example 6-1]
Electrolytic solutions were prepared by changing the type of solvent, and cyclic voltammetry measurement was performed using each electrolytic solution. Specifically, the solvents in the production examples are (a) 2-methyltetrahydrofuran (ether solvent; manufactured by Tokyo Chemical Industry), (b) methylisopropylsulfone (MiPS; sulfone solvent; manufactured by Tokyo Chemical Industry), (c) sulfolane ( The electrolyte solution was prepared by changing to a sulfone solvent (manufactured by Tokyo Chemical Industry Co., Ltd.), and cyclic voltammetry measurement was performed in the same manner as in Example 1. However, the measurement of the electrolytic solution using 2-methyltetrahydrofuran as a solvent was performed in an extremely low humidity environment using a glove box.

(result)
Each cyclic voltammogram is shown in FIGS. 7A to 7C. From these figures, it can be seen that even when the electrolyte was prepared by changing the solvent, the potential at which magnesium dissolution and precipitation began did not change. It can also be seen that no overvoltage condition has occurred. This fact suggests that the structures of magnesium complexes related to magnesium dissolution and precipitation are similar (presumed to be 4-coordinated by solvent molecules).

[Example 6-2]
Electrolytic solutions were prepared by changing the single halogen to bromine molecules and changing the type of solvent, and cyclic voltammetry measurement was performed when each electrolytic solution was used. Specifically, the iodine in the production example was changed to bromine (5 g; manufactured by Wako Pure Chemical Industries), and the solvent was further changed to (a) DEMETFSI (Diethylmethyl (2-methoxyethyl) ammoniumbis (trifluoromethylsulfonyl) imide; ionic liquid; manufactured by Kishida Chemical Co., Ltd.) ), (B) methyl isopropyl sulfone (MiPS; sulfone solvent; manufactured by Tokyo Kasei), (c) dipropyl sulfone (DnPS; sulfone solvent; manufactured by Tokyo Chemical Industry) (d) sulfolane (sulfone solvent; manufactured by Tokyo Chemical Industry) The electrolyte solution was prepared by changing to, and cyclic voltammetry measurement was performed in the same manner as in Example 1.

(result)
Each cyclic voltammogram is shown in FIG. 8 (a) to (d). From these figures, even when the electrolyte was prepared by changing the simple halogen and the solvent, the potential at which magnesium was dissolved and precipitated was almost the same as in Example 1 when a solvent other than DEMETFSI was used. Similarly, it can also be seen that when a solvent other than DEMETFSI is used, no overvoltage condition has occurred. Further, even when DEMETFSI is used as a solvent, a response current is generated from about −0.5 V, and the dissolution of magnesium starts at an earlier stage than the prior art (see, for example, FIG. 1B). It will be. This fact suggests that the structures of magnesium complexes related to magnesium dissolution and precipitation are similar (presumed to be 4-coordinated by solvent molecules).

The present invention can be used for, for example, a magnesium secondary battery.

Claims

Electrolytic solution made by mixing solvent, metallic magnesium and simple halogen.
The electrolytic solution according to claim 1, comprising a solvent, magnesium ions and halide ions.
The electrolytic solution according to claim 1 or 2, which contains 0.5 mol / L or more of magnesium ions with respect to the total amount of the electrolytic solution.
The number of magnesium atoms having a coordination number of 4 occupies 95% or more of the total number of magnesium atoms when analyzed by a soft X-ray fluorescence XAFS method. Electrolyte of description.
The electrolyte solution according to any one of claims 1 to 4, wherein the solvent is an organic solvent or an ionic liquid.
6. The electrolytic solution according to claim 1, wherein the solvent is a sulfone solvent.
The electrolytic solution according to any one of claims 1 to 6, which contains bromide ions or iodide ions as halide ions.
A secondary battery containing the electrolytic solution according to any one of claims 1 to 7.
An electrolytic solution manufacturing method including a step of mixing a solvent, metallic magnesium, and a simple halogen.
10. The method for producing an electrolytic solution according to claim 9, comprising 0.5 mol / L or more of magnesium ions with respect to the total amount of the electrolytic solution.
The electrolysis according to claim 9 or 10, wherein when the electrolytic solution is analyzed by a soft X-ray fluorescence XAFS method, magnesium atoms having a coordination number of 4 occupy 95% or more of the total number of magnesium atoms. Liquid manufacturing method.
The method for producing an electrolytic solution according to any one of claims 9 to 11, wherein the solvent is an organic solvent or an ionic liquid.
The method for producing an electrolytic solution according to any one of claims 9 to 12, wherein the solvent is a sulfone solvent.
The method for producing an electrolytic solution according to any one of claims 9 to 13, wherein the single halogen is a bromine molecule or an iodine molecule.